Human influenza 人類流感
Influenza is a respiratory infection caused by the influenza virus, different forms of which are normally circulating in not just human beings but pigs and birds. All influenza viruses are divided into A, B and C, and further distinguished by their H and N antigens. There are currently 16 H (standing for haemaglutinin) and 9 N (standing for neuraminidase) antigens. The novel swine flu discovered in 2009 has probably originated from pigs and is classified as Influenza A (H1N1). The genetic structure of the novel swine flu contains segments from pig, bird and human, reflecting the occurrence of reassortment. Studies suggested a case fatality rate of 0.4% for the novel infection, and a higher tendency for causing disease in young people compared to seasonal flu.